The New York attorney-general is preparing to file a civil lawsuit against Credit Suisse for misleading investors who lost billions of dollars on mortgage-backed securities, according to a source familiar with the matter.

U.S. District Judge Katherine Forrest in Manhattan, who took over the case from her colleague Barbara Jones last month, rejected the SEC's argument that a recent court decision made a $150m note sale to Germany's IKB Deutsche Industriebank AG sufficiently 'domestic' to give her jurisdiction.

Monday's decision does not affect the rest of the SEC's lawsuit against Tourre, which arose from charges filed against him and Goldman in April 2010.